1. Field of the Invention
This invention relates to zoom lenses suited to photographic cameras or video cameras and, more particularly, to zoom lenses having three lens groups of which the first counting from front is of negative power and in which these three lens groups are moved to vary the image magnification. Still more particularly, it relates to zoom lenses of high range with a short total length and excellent optical performance.
2. Description of the Related Art
The zoom lens having three lens groups of which the first counting from front is of negative power, the second is of positive power and the third is of negative power and in which at least two of these are made to move to vary the image magnification has been proposed in, for example, Japanese Laid-Open Patent Applications Nos. Sho 55-11260 and Sho 56-159614, Japanese Patent Publication No. Sho 58-50327 and British Patent No. 398307.
This type of zoom lens, because being relatively amenable to increase the angle of view, has been utilized in many wide-angle photographic systems.
However, the zoom lenses of Japanese Patent Publication No. Sho 58-50327 and British Patent No. 398307 have a relatively long total length (the distance from the front vertex to the image plane) and their front lens members have also a large diameter. To achieve a minimization of the bulk and size of the camera, they are not always adequate.
The zoom lenses of Japanese Laid-Open Patent Applications Nos. Sho 55-11260 and Sho 56-159614 are adapted to be objective lenses for copy machines and have no positive aim to vary the focal length. These zoom lenses each are used in such a way that the lens system is inverted as a whole as the magnification changes between lower and higher values, or operates two modes in so-called conjugate relation. The values of the focal length at the minimum and maximum magnifications are nearly equal to each other, and the focal length is made to vary in a range between this value and its half. Hence, these zoom lenses are not suited to photographic cameras, video cameras, etc..
Apart from these zoom lenses, a zoom lens of the same type as that described above but suited to the photographic camera, or video camera is proposed in a Japanese Laid-Open Patent Application No. Sho 58-200208. In the same document, of the three lens groups of minus-plus-minus power arrangement in this order from the front, the first and second groups are moved in a particular relation when performing variation of the magnification, and particular rules of design for the three lens groups are set forth. Owing to these features, a zoom lens well corrected for good stability of aberrations against zooming, or having high optical performance is achieved. As the related art, there is U.S. patent application Ser. No. 187,277 filed on Apr. 28, 1988.